Plasma arc torches are widely used for cutting metallic materials. A plasma arc torch generally includes a torch body, an electrode mounted within the body, passages for cooling and arc control fluids, a swirl ring to control the fluid flow patterns, a nozzle with a central exit orifice, electrical connections, and a power supply. The torch produces a plasma arc, which is a constricted ionized jet of a plasma gas with high temperature and high momentum. A shield may also be employed to provide a shield gas flow to the area proximate the plasma arc. Gases used in the torch can be non-reactive (e.g. argon or nitrogen), or reactive (e.g. oxygen or air).
In operation, a pilot arc is first generated between the electrode (cathode) and the nozzle (anode). The pilot arc ionizes gas passing through the nozzle exit orifice. As the ionized gas reduces the electrical resistance between the electrode and the workpiece, the arc transfers from the nozzle to the workpiece. The torch may be operated in this transferred plasma arc mode, which is characterized by the conductive flow of ionized gas from the electrode to the workpiece, for the cutting of the workpiece.
Generally, there are two widely used techniques for generating a pilot plasma arc. One technique uses a high frequency, high voltage ("HFHV") signal coupled to a DC power supply and the torch. The HFHV signal is typically provided by a generator associated with the power supply. The HFHV signal induces a spark discharge in the plasma gas flowing between the electrode and the nozzle, and this discharge provides a current path. The pilot arc is formed between the electrode and the nozzle with the voltage existing across them.
The other technique for generating a pilot plasma arc is known as contact starting. Contact starting is advantageous because it does not require high frequency equipment and, therefore, is less expensive and does not generate electromagnetic interference. In one form of contact starting, the electrode is manually placed into physical and electrical contact with the workpiece. A current is then passed through the electrode to the workpiece and the arc is struck by manually retracting the electrode from the workpiece.
Improvements in plasma arc torch systems have been developed which have eliminated the need to strike the torch against the workpiece in order to initiate an arc, thereby avoiding damage to brittle torch components. One such system is disclosed in U.S. Pat. No. 4,791,268 ("the '268 patent"), which is assigned to the same assignee as the instant invention. Briefly, the '268 patent describes a torch having a movable electrode initially in contact with a stationary nozzle due to a spring coupled to the electrode such that the nozzle orifice is blocked. To start the torch, current is passed through the electrode and nozzle while a plasma gas is supplied to a plasma chamber defined by the electrode, the nozzle, and a swirl ring. Contact starting is achieved when the buildup of gas pressure in the plasma chamber overcomes the spring force, thereby separating the electrode from the nozzle and drawing a low energy pilot arc therebetween. Thereafter, by bringing the nozzle into close proximity with the workpiece, the arc may be transferred to the workpiece, with control circuitry increasing electrical parameters to provide sufficient energy for processing the workpiece. Plasma arc torch systems manufactured according to this design have enjoyed widespread acceptance in commercial and industrial applications.
During operation of a plasma arc torch, a significant temperature rise occurs in the electrode. In systems which employ a movable electrode, passive conductive cooling of the electrode by adjacent structure is reduced due to the need to maintain sliding fit clearances therebetween. Such clearances reduce heat transfer efficiencies relative to fixed electrode designs employing threaded connections or interference fits. Accordingly, active cooling arrangements have been developed such as those disclosed in U.S. Pat. No. 4,902,871 ("the '871 patent"), which is assigned to the same assignee as the present invention. Briefly, the '871 patent describes an electrode having a spiral gas flow passage circumscribing an enlarged shoulder portion thereof Enhanced heat transfer and extended electrode life are realized due to the increased surface area of the electrode exposed to the cool, accelerated gas flow.
While known contact starting systems function as intended, additional areas for improvement have been identified to address operational requirements. For example, in known contact starting systems, the electrode is supported in part by a spring which maintains intimate electrical and physical contacts between the electrode and nozzle to seal the exit orifice until such time as the pressure in the plasma chamber overcomes the biasing load of the spring. Degradation of the spring due to cyclic mechanical and/or thermal fatigue lead to change of the spring rate or spring failure and, consequently, difficulty in initiating the pilot arc with a concomitant reduction in torch starting reliability. Accordingly, the spring should be replaced periodically; however, due to the location of the spring in the torch body, additional disassembly effort is required over that necessary to replace routine consumables such as the electrode and nozzle. A special test fixture will typically also be needed to assure proper reassembly of the torch. Further, during repair or maintenance of the torch, the spring may become dislodged or lost since the spring is a separate component. Reassembly of the torch body without the spring or with the spring misinstalled may result in difficulty in starting or extended operation of the torch prior to pilot arc initiation.
Additionally, sliding contact portions of the electrode and proximate structure, which may be characterized as a piston/cylinder assembly, may be subject to scoring and binding due to contamination. These surfaces are vulnerable to dust, grease, oil, and other foreign matter common in pressurized gases supplied by air compressors through hoses and associated piping. These contaminants diminish the length of trouble free service of the torch and require periodic disassembly of the torch for cleaning or repair. It would therefore be desirable for moving components and mating surfaces to be routinely and easily replaced before impacting torch starting reliability.